Castell Mawr is a large limestone rock on the western shore of Red Wharf Bay in

Anglesey Anglesey ( ; ) is an island off the north-west coast of Wales. It forms the bulk of the Principal areas of Wales, county known as the Isle of Anglesey, which also includes Holy Island, Anglesey, Holy Island () and some islets and Skerry, sker ...

, north

Wales Wales ( ) is a Countries of the United Kingdom, country that is part of the United Kingdom. It is bordered by the Irish Sea to the north and west, England to the England–Wales border, east, the Bristol Channel to the south, and the Celtic ...

. It was probably once the site of an

Iron Age The Iron Age () is the final epoch of the three historical Metal Ages, after the Chalcolithic and Bronze Age. It has also been considered as the final age of the three-age division starting with prehistory (before recorded history) and progre ...

fort, but today the rock is a protected refuge for nesting seabirds. It is located between the village of Red Wharf Bay and the promontory where St. David's Holiday Park is sited. The closest large villages are

Pentraeth Pentraeth () is a village and Community (Wales), community on the island of Anglesey (''Ynys Môn''), North Wales, at . The Royal Mail postcode begins LL75. The community population taken at the United Kingdom Census 2011, 2011 census was 1,178 ...

and

Benllech Benllech () is a village on the Isle of Anglesey in Wales. It is in the community of Llanfair-Mathafarn-Eithaf, which has a population of 3,382. The name of Benllech village had been removed by the time of the 2011 census with the community b ...

. The site is notable for being the only location on Anglesey displaying genuine

karst Karst () is a topography formed from the dissolution of soluble carbonate rocks such as limestone and Dolomite (rock), dolomite. It is characterized by features like poljes above and drainage systems with sinkholes and caves underground. Ther ...

development in a series of small caves and solutional cavities. Castell Mawr was selected by the

Countryside Council for Wales The Countryside Council for Wales (CCW; ) was a Welsh Assembly sponsored body responsible for wildlife conservation, landscape and countryside access in Wales. It merged with Forestry Commission Wales, and Environment Agency Wales to form ...

because of its geological interest, as it provides exposures of carboniferous limestone strata. The rock consists of limestone and sandstone layers, part of the Benllech Limestone formed during the Brigantian Stage of the lower Carboniferous period. The limestone was once mined and exported to other centres in the UK and Ireland.

References

{{coord, 53.3103, -4.2061, type:landmark_region:GB, display=title Mountains and hills of Anglesey Pentraeth